The present invention is generally related to abrasive compositions for use as drilling or cutting compounds. More particularly, this invention is related to abrasive slurries for use with very hard materials, particularly boron carbide.
Drilling or machining of solid boron carbide (B.sub.4 C) is well known to be difficult and time consuming due to the hardness of boron carbide. On the revised Mohs scale, wherein diamond is assigned a hardness of 15, boron carbide has a hardness of from 12 to 15, depending on such factors as the stoichiometric purity, density and microstructure of a particular sample of boron carbide. In view of this hardness, diamond grit would ordinarily be selected for use as the abrasive grit in a drilling or cutting slurry for boron carbide. However, when diamond grit is used as an abrasive in conventional slurries it is found that there is a tendency for the workpiece to "wipe," that is, for a mass of boron carbide to adhere in a smooth layer to the cutting tool, thereby rendering the cutting tool useless. It is well known to use various types of abrasive slurries in the machining of hard materials such as boron carbide. It has been found that the effectiveness of any particular abrasive slurry for the purpose of drilling or machining boron carbide is critically dependent on the viscosity, volatility and surfactant characteristics of the carrier solution in which the abrasive grit is suspended. In this regard, it has been found that small variations in these characteristics can result in large variations in the time required to perform a particular drilling task. If the carrier solution is too viscous it does not flow with sufficient fluidity to adequately penetrate between the contacting surfaces of the cutting tool and the workpiece, with the result that the workpiece and tool are inadequately lubricated and quickly overheat. Conversely, if the carrier solution is too thin the abrasive grit tends to settle out and cutting action is impaired. Also, if the solution is too thin it is ordinarily relatively volatile and has a tendency to volatilize at localized points where frictional heat is generated, a problem which is self-accelerating inasmuch as there is created a dry hot spot, which quickly generates substantially more frictional heat and leads to damage of the cutting tool and/or the workpiece. Further, the surfactant characteristics of the carrier solution are important because they determine the capability of the carrier solution to wet both the workpiece and the cutting tool, and thus affect the ability of the slurry to penetrate and lubricate the contacting surfaces of the workpiece and the cutting tool.